Beat butter on high speed with an electric mixer for about three minutes until light and airy.
Mix in confectioners' sugar (powdered sugar) a little bit at a time.
Add vanilla, salt, and sour cream and continue to beat for another minute. If desired, add more more confectioners' sugar to make the buttercream frosting stiffer.
Notes
This frosting frosts about 12 cupcakes with a large swirl or an 8" square cake.